8 Ways to Make Vampires Realistic

3. Feeding

Blood is rich in proteins and fats, and many creatures live entirely off a diet of blood. These hematophages, such as the vampire bat, tend to be parasites. While it may seem a natural fit to make the homo vampiris a parasite like in The Strain, that doesn't quite fit the picture we're building here.

A parasite is generally much smaller than its host and will often live or feed on its host for an extended period of time - without killing it. A lamprey, for instance, can feed on a shark for a long time without risking its host's death. Relative to the shark, the smaller lamprey requires a lesser amount of nourishment to survive. According to Kleiber's law, metabolic rate scales according to body mass. In other words, the bigger a creature, the more energy it needs.

The energy content of blood is about 900 calories per liter, and the human body holds roughly 5.5 liters of blood. A human needs about 2,000 daily calories to maintain its weight, and as per Kleiber's law, a vampire would have roughly the same requirements, which comes out to 2.2 liters of blood per day.

How much blood can a human afford to lose before it dies? A healthy person can endure up to 15% blood loss without serious concern, up to 30% blood loss with visible skin pallor, and up to 40% while going into shock. Beyond 40% blood loss, the person will die unless extreme resuscitative measures are taken.

The math just so happens to work out that 40% of 5.5 liters is 2.2 liters, so for a vampire to get his daily blood requirement from a human, he'd have to bring him to the brink of death. Alternatively, he can safely feed on two or more humans.

But here's the stickler: blood isn't a nutritionally complete substance. It lacks sufficient vitamins, minerals, fiber, carbohydrates, and essential fatty acids to be a balanced food. Vampire bats are able to live off blood alone, but in order to get all the nutrients they need, they consume an incredible amount of blood relative to their body weight. A typical bat can drink over half of its weight in blood in a single feeding. For a 150 lb vampire, that would be about 75 lb of blood, which works out to roughly 38 liters - seven humans worth of blood.

While we can try to work out rationales that explain how vampires can sustain this degree of food intake without wiping out vast swaths of human population, it's much easier to simply drop the concept that vampires are parasites entirely - they're predators. Parasites evolved means of feeding that go largely unnoticed by their hosts because the host would otherwise easily be able to overpower the parasite. A vampire is stronger than a human, so it has no need to be discrete. A lion doesn't quietly nibble on a gazelle's thigh - it savagely rips out its throat.

Given a vampire can easily subdue a human, there's no reason we can't have our homo vampiris supplant its diet with flesh. In fact, many hematophages don't exclusively rely on blood as their sole food source. Blood can still remain a vital part of the feeding process, but by getting some flesh in there as well, we're cutting down tremendously on the amount of blood - and thus humans - a vampire needs a day to survive.

And why only human blood? Well, we'll have to relegate that to societal reasons. In human culture, eating other humans is taboo. Maybe in our vampire culture, feeding on anything other than a human is taboo.